Recently, in the machinery field, a fiber-reinforced resin material containing carbon fibers and a thermoplastic resin has come to receive attention, and in joining fiber-reinforced resin materials necessary in production of parts and structures, a method of increasing rigidity by forming a closed cross section has been proposed. In joining fiber-reinforced resin materials using a thermoplastic resin as the matrix therein, mechanical fastening using bolts, nuts, rivets or the like, chemical joining using an adhesive, and thermal joining using ultrasonic welding, vibration welding or the like has heretofore been proposed, and above all, ultrasonic welding is widely used in various industrial fields for the reason that it does not require any third material and additionally the cycle time thereof is short.
Ultrasonic welding is a method of welding a workpiece to another workpiece that is in contact therewith, in which a resonator called a welding horn is pressed against workpieces while high-frequency mechanical vibration is given from the resonator, and the mechanical vibration transferred to the workpieces is converted into frictional heat to thereby heat and fuse a part of the workpieces.
Ultrasonic welding includes, as roughly classified, two techniques. One is a welding method through ultrasonic application in direct contact with a fiber-reinforced resin material (Patent Document 1), and another is a welding method where projections called energy directors are formed on the surface of a fiber-reinforced resin material and the energy directors are intensively vibrated and fused in ultrasonic application (Patent Document 2, Patent Document 3).
In general, the former method is used for welding thin materials such as film laminates or the like, but Patent Document 1 describes an invention in which a hat-shaped molded article is produced from an isotropic base material containing carbon fibers and a thermoplastic resin, and a pair of the molded articles are ultrasonically spot-welded to form a tubular joined body.
The latter method may be used in welding samples thicker than films, in which, however, the time for ultrasonic application tends to be long. This is because the ultrasonic mechanical vibration attenuates inside the resin and therefore the ultrasonic mechanical vibration could not be utilized efficiently for welding.
Recently, it has become desired to efficiently produce a joined body more excellent in joining strength.